Aluminum-manganese-iron alloys



Nov. 19, 1963 R. E. CAIRNS, JR., ETAL 3,111,405 ALUMINUM-MANGANESE-IRON ALLOYS Filed June 16, 1958 I292 I382 I472 I562 I652 I742 I832 Annealing Temperaiure F I292 I382 I472 I562 I652 I742 I832 Annealing Tempera'l'ure F I292 I382 I472 I562 I652 I742 I832 Annealing Temperal'ure F INVENTORS R ymaui C r s fl. BY

\IO \V\ HAM 3,111,405 ALUMlP-IUM-MANGAJ IESE-ERN ALLOYS Raymond E. Cairns, .112, Needham, and .iohn 1.. Han], Wellesley Hills, Mass, assignors, by mesne assignments, to Langley Alloys Limited, a company of Great Britain Filed ions 16, 1958, Ser. No. 742,410 7 Claims. (Cl. 75-124) This invention relates to the production of metals and more particularly to alloys for elevated temperature service.

A principal object of the present invention is to provide low density, oxidation resistant, cold-ductile alloys from non-strategic materials.

Another object of the invention is to provide alloys of the above type which can be fabricated by normal steel processing techniques.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawing which is a graph showing the effect of cold rolling and subsequent heat treating of certain alloys.

The interest in iron-aluminum alloys dates back to early 1930 when Sykes and Bampfyalde in Journal Iron and Steel Institute, volume 130, 1934, pp. 389-418 discussed the physical properties of iron-aluminum alloys. Recently certain iron-aluminum alloys (Alfenols) have become the center of interest because of their excellent magnetic and high temperature properties. Such alloys and their propeities are described in detail in the following publications.

The Fabrication and Properties of 16-AlfenolA New Strategic Aluminum-iron Alloy, by J. F. Nachrnan and W. J. Buehler, Naval Ordnance Report 2819, April 9, 1953; 16 Percent Al-le Alloy Cold Rolled in the Order-Disorder Temperature Range, by 1. F. Nachman and W. I. Buehler, Journal of Applied Physics, 25, No. 3, 307,212, March 1954; Applications and Properties of Iron10-17% Aluminum Alloys, by J. F. Nachrnan and W. I. Buehler, Naval Ordnance Report 4130, December 5, 1955; Rolling of High Aluminum-iron Alloys, by K. Foster and D. Pavlovic, WADC Technical Note 56-29, December 1955; Aluminum in Iron and Steel, by S. L. Case and K. R. Van Horn, John Wiley and Sons, inc. (1953); Metals Handbook, American Society for Metals, 1948, pp. 116. The Alfenols possess excellent resistance to oxidation and have a corrosion resistance comparable to that of 18-8 stainless steel.

Nachman and Buehler added two to four percent molybdenum to a ten to eighteen percent aluminum-iron alloy and called it Thermenol. Their work is described in U.S. Patent 2,768,915; Applications, Properties and Fabrication of Thermenol Type Alloys, by 3. F. Nachman and W. l. Buehler. Naval Ordnance Report 4237, May 4, 1955; and ThermenolA new Soft Magnetic Material, by I. R. Nachman and W. I. Buehler. Electrical Manufacturing, November 1956, pp. 140-145.

Thermenol was found to have superior mechanical properties to 403 stainless steel above 800 F. and a better strength-weight ratio for intermediate temperature than many common aircraft structural materials.

Various properties and alloy additions to Alfenol and Thermenol base alloys were discussed in U.S. Patents 2,726,952 and 2,804,387; and in Ductile Iron-Aluminum Alloys, by E. R. Morgan and W. F. Zackay, Metal Progress, 68 No. 4, pp. 126, October 1955 and Informal NOL Progress Report, January 6, 1957, Bauer Allotment No. 85680, Amendment 1, January 6, 1956.

Although the above mentioned specific iron-aluminum rates tent 3,ll1,4- Patented Nov. 19, 1963 alloys have good oxidation resistance, corrosion resistance, and are relatively reasonable in cost, one serious limitation thereof is their lack of cold ductility which makes them diflicult to process and form at room temperature.

In U.S. Patents 2,329,186 and 2,376,869, it was shown hat some ductility could be achieved by the addition of 20 to 50 percent manganese to iron-aluminum alloys. In the present invention there are provided modified binary iron-aluminum alloys which possess not only good oxidation resistance but also increased room temperature and 1200 F. strength, decreased density and good cold ductility and workability.

The alloys of the present invention comprise by weight 20 to 40 percent manganese, 7 to 16 percent aluminum, 0.15 to 1.1 percent carbon, 0 to percent nickel and the balance substantially iron. The superior properties or these alloys is achieved by controlling the amounts of manganese, carbon and aluminum so as to assure the presence of austenite in amounts greater than about percent in the alloy.

In another embodiment of the invention, the hardness of the above alloys is improved by cold rolling to achieve a reduction of at least percent and then heat-treating the cold worked alloy at a temperature between about 1110 F. and 1740 F.

It has been found that by changing the crystal structure of the alloys from entirely body-centered cubic, which is normally associated with iron-aluminum alloys, to alloys containing at least 15 percent of face-centered-cubic structure, vastly improved properties resulted. As Will be shown, the alloys of the instant invention have excellent workability, good room temperature and 1200 F. strength, good room temperature ductility and oxidation resistance at high temperatures. Additionally they can be hardened by a process of cold reduction and heat treatment so that substantial improvement in short and long term strength can be obtained. Furthermore, the present alloys have densities significantly lower than that of stainless steel and other high temperature alloys. The instant alloys, because of their unique combination of properties, make them useful for applications such as skins for missiles and rockets, general aircraft structural components and many other areas where high strength sheet and bar material are required.

A number of iron-aluminum alloys were melted and cast in a vacuum-induction furnace. The chemical composition of the more significant experimental alloys, along with two commercial iron-aluminum alloys, is given in ice Table 1.

TABLE 1 Volume Percent Percent Percent Percent Percent Face-Center- Alloy N0. Al 0 Mn Other Cubic Phase Contained in the Structure 133 1 16. 4 0.02 Mo2.5 O 134 14. 5 0. 04 Ti2.9 0 136 9. 4 0.05 20. 6 10 172 15. 3 1. 08 28. 3 20 173 12. 2 0. 67 25. 2 179" 7.0 0.03 39. 3 100 196 16. 5 0. 10. 4 1VIO2.8 0 210-.-" 14. 6 0.04 L111 O.22 0 213--- 10.2 0.76 100 214.. 7. 5 0.79 100 226 10. 0 0. 27 227- 12. 5 0. 30 228 9. 3 0. 34

1 Commercial iron-aluminum alloys. 2 lviisch metal.

As noted in the literature and confirmed here, binary iron-aluminum alloys although ductile when hot, are hard. very brittle at ordinary temperatures, and impossible to a fabricate by many of the normal steel processing techiques. It is also well known that body-centered-cubic materials undergo a transition at a particular temperature where the nature of the failure of the material changes 4 1200" F. The alloys, in general, were subjected to two or three heat treatments prior to testing. The heat treatment along with the tensile-testing results for the alloys listed in Table 1 are given in Table 2.

TABLE 2 Alloy No. Percent Percent Percent Percent Percent Test Heat Tensile Yield Percent Percent Al Mn Other Austenite Temp. Treat Sgth. Sgth. E R. A.

133 16.4 0.02 BTO Zfi..- 0 IlOOOO 6:666-

134 14.5 0.04 Ti2.9 0 {Eg 2 RT 79,500 2 5 136 94 0.05 206 8 8 1,200 $6, 18;! 99 RT 1 2, .5 2.4 172 L08 20 {1,200 12,000 166 93 AT 14 14 173 12.2 0.07 25.2 2 1,200 45, 000 22 24 RT 28,500 72 179 70 0.03 39.3 100 22,500 57 72 1,200 13, 000 88 60 RT 69,000 0 0 106 16.5 0.55 10.4 Mo-"8... 0{ 5,000 0.5 0.3

1,200 12,000 62 65 RT 102, 750 1. 0 2. 0 210 14.6 0.02 l\[M0.22 0 1.5 1.0

1,200 21,000 92 37 RT 55,000 73 71 213 10.2 0.76 34.4 100 81,000 72 82 1,200 40, 500 34 32 I T 51,000 62 73 214 7.5 0.79 37.0 100 54,750 66 71 11,200 33,000 65 64 T 65,500 45 51 220 10.0 0.27 33.3 Ni-2.3 60 49,500 25 30 1,200 27, 500 51 41 RT 63,750 45 52 227 12.5 0.30 33.0 Ni 1.3-- 55,500 35 32 1,200 30,000 37 31 RT 63,250 43 39 22s 9.3 0.34 35.4 Ni6.4- 52,500 35 32 1,200 36,000 31 31 1 RT=Room Temperature; 1200=1200 F.

2 The number is the temperature in degrees F. that the alloy Was held at prior to cooling.

The suffix describes the manner in which the alloy was cooled: Q,=hot oil quench; FC=furnace cool.

3 Sheet Specimen NAVORD Report 4237 Applications, Properties and Fabrication of Thermenol Type Alloys.

from a brittle to a ductile nature. Thus one reason for the brittle nature of iron-aluminum alloys is that room temperature is considerably below the brittle to ductile transition temperature. This effect was found to be conteracted by changing the crystal structure of the alloys from body-centered-cubic (ferrite) to face-centered-cubic (austenite), a structure which does not undergo the aforementioned transition change. Large amounts of carbon and manganese are needed to stabilize the face-centeredcubic phase, since aluminum promotes the body-centeredcubic structure in iron. The actual effects of carbon and manganese on the crystal structure can be seen in Table 1. It was found, initially, that austenite in amounts greater than 15 percent improved the machinability characteristics to a considerable degree. Speeds and feeds normally associated with stainless steel and which are normally considered too high for ferritic iron-aluminum alloys could be used without difiiculty. It had been thought by other investigators that the large grained structure which is normally associated with iron-aluminum alloys was the chief cause of the poor machinability characteristics. However, alloy 210, a percent ferritic alloy that possessed a small grain size by virtue of the grainrefining eficct of 0.22 percent misch metal, possessed machinability characteristics that, although slightly better than the coarse grained ferritic alloys, were inferior to the alloys containing greater than 15 percent austenite.

The 'for geability characteristics of both the fer-ritic alloys and the alloys containing austenite are very good. In most instances, a fairly wide range of temperatures (2000 to 2300 F.) could be used without harmful effects. The more significant experimental alloys were forged to either %-inch diameter or %-inch square rod suitable for obtaining specimens for mechanical testing. The alloys listed in Table 1 along with others were subjected to tensile testing at both room temperature and Discussion of T ensile-T esting Results In general, the results of the tensile-testing program were quite good, and it was found that by changing the crystal structure from body-centered-oubic (ferrite) to face-centcred-cubic (austenite) that not only was the room temperature and 2100 F. tensile strength increased, but the room-temperature ductility was improved by two orders of magnitude. The actual effect of austenite on the elongation at room temperature can be clearly seen in Table 3.

The elongation values of Tables 2 and 3 are based on Z-inch gauge lengths and a 0.2 percent offset was used except for sharp yield points where no offset was necessary.

The amount of austenite present in an alloy was found to be the significant factor, the ductility increasing as the amount of austenite increased. Carbon was also found to be a significant factor on the tensile strength. The highest carbon concentration attained was 1.08 percent, resulting in an alloy having a room-temperature tensile strength of 168,000 p.s.i. and a 1200 F. tensile strength of 74,250 p.s.i. In addition, carbon in appreciable concentrations refines the grain structure to a considerable degree. for each experimental alloy in the following paragraphs:

Alloy 133.The tensile-test data obtained for this alloy did not give a true picture of its tensile strength either at room temperature or 1200 F. Therefore, the reported values of others are also listed in Table 2. Even when the most careful machining techniques were tried on the tensile specimens, the large grains pulled out from the surface leaving sharp-edged indentations. These indentations naturally lowered the tensile strength by a considerable degree. The 2000 F. quench heat treatment was drastic, and the specimens tested in this condition failed under considerably less stress than when tested in furnace-cooled condition. These results emphasize the difiiculties encountered in Thermenol when subjected to machining operations.

Alloy 134.The tensile-test data for this alloy is quite similar to that of alloy 133. The same machinability characteristics were encountered, and all but one specimen exhibited surface defects. The 1200 F. tensile test on a specimen quenched from 1450 F. should be considered valid and tensile strength of 47,250 p.si. should be considered as quite good for this type of alloy. The room-temperature tests again show that the 2000 F. quenched specimens failed under a lower stress than furnace cooled specimens.

Alloy 136.This alloy contains approximately percent austenite at room temperature. Although this alloy does not have exceptional strength at either room temperature or 1200 B, it can be seen, however, that tensile properties of this alloy can be varied to a large degree by heat treatment. This is the only alloy in the tensiletesting program where heat treatment results in such a. noticeable effect.

The 2000 F. quench heat treatment was too drastic and the specimen failed under a very small stress. A metallographic examination of this specimen showed internal cracking in individual grains. After heat treating at 1450 F. and quenching, the alloy showed good tensile strength and an excellent degree of ductility. A metallographic examination showed that a second phase was precipitating throughout the matrix and along the grain boundary. This phase was austenite. When specimen is furnace cooled from 2000 F., both the tensile strength and the ductility decrease. The microstructure still con sisted of two phases, but the austenite has precipitated only along the grain boundaries. This phenomenon can be observed in the 1200 F. tensile results also.

The oxidation resistance of this alloy is very good at 2200 F., and is comparable to that of alloy 133. The machinability characteristics, although not as good as some of the experimental alloys, is definitely superior to that of the 100 percent ferritic alloys.

Alloy 172.-Alloy 172 contains approximately 20 percent austenite, and exhibits a very fine-grain structure due to a rather high carbon concentration (1.08 percent). The machinability characteristics where vastly improved so that speeds and feeds normally associated with stainless steel could be used. The tensile strengths both at room temperature and 1200" F. are the highest of any iron-aluminum alloy tested or referred to in the literature (168,000 p.s.i. at room temperature and 74,250 p.s.i. at 1200 F.). Some room-temperature ductility was observed on the 2000 F. quenched specimen (4.5 percent elongation and 2.4 percent reduction in area), which is a definite improvement over the normal ductility in a 0.505- inch diameter, ironpercent aluminum alloy. The yield strength of the specimens tested at 1200 F. is low when compared to the other experimental alloys and the elongation and reduction in area is quite large. This large elongation and reduction in area was found to be quite common in the ferrite iron-aluminum-manganese alloys as can be seen by a perusal of the tensile-test data.

The interesting fact concerning this alloy and other alloys containing carbon in amounts reater than 0.5

The tensile-test results are discussed in detail percent is that the machinability and hot workability are not adversely effected. In many references in the literature, carbon has been referred to as having a detrimental effect on the ductility of iron-aluminum alloys.

Alloy 173.Alloy 173 contains approximately 40 percent austenite, and has a fine-grained structure which is quite similar to alloy 172. The aluminum, carbon, and manganese concentrations are lower in this alloy than in alloy 172, and although the high carbon conccntration (-0.67 percent) is not as high, it has the same effect on the grain structure. Both the room temperature and the 1200 F. tensile-test results are very good. In the room-temperature test on the specimens quenched from 2000 F, not only is the tensile strength extremely high, but the ductility has been increased by one order of magnitude over a comparable iron-12 percent aluminum alloy. The tensile strength and ductility decreased, when the specimens were furnace cooled because the amount of austenite decreased. The 1200 F. tensile strength is not as hi h as that recorded for some of the alloys that will be discussed in following paragraphs, but is quite good. The yield strength at 1200 F. (45,000 p.s.i.) shows an increase over alloy 172, and is one of the highest recorded in the program. The elongation and reduction in area in the 1200 F. test show a decrease as one would except with an increased yield strength, and this is at tributed to the smaller amount of austenite being present.

Alloy 179.-This alloy is one of the few alloys that is completely austenitic, and is the only alloy that attained this condition through the addition of manganese alone (0.03 percent carbon). The other completely austenitic alloys contain at least 0.70 percent carbon. The roomtemperature tensile strength is not very high (72,000 p.s.i.) but the elongation and reduction in area are excellent, 57 and '72. percent, respectively. This is a tremendous increase in room-temperature ductility over ferritic iron-aluminum alloys. As one would expect in an austenitic alloy, the tensile properties are exactly the same for the two heat treatments. The 1200" F. strength is average for the alloys tested in this program. An interesting comparison can be made between this alloy and an alloy almost identical in chemical composition, except for a high carbon concentration. This comparison is made in the discussion of alloy 214.

Alloy 196.This alloy is percent ferritic, but contains 10 percent manganese and a high carbon concentration of 0.55 percent. The grain structure of this alloy is very fine and this fact alone improves the machinability. The condition of the surface of the specimen prior to testing is good. The alloy was not tested in the 2000." F.-quenched condition, since internal cracking might result as it had in previous alloys. The room-temperature tensile strengths are not exceptional, and the chief difference between the tensile strengths of the quenched and the furnace-cooled specimens is attributed to ordering taking place during the furnace-cooled operation. Carbon does not seem to increase the room-temperature tensile strength as it does in some of the other experimental alloys. The fine-grain size does not improve the ductility by itself, and cannot, therefore, be considered to be the chief cause of the ductility in other experimental alloys.

The 1200 F. tensile strength is excellent (62,500 p.s.i.) and is only exceeded by that of alloy 172. Again, as in the case with alloy 172, the yield strength is low and in this case one of the lowest obtained in the program. Carbon though ineffective at room temperature in this alloy, seems to be the chief cause of the high tensile strength at 1200 F.

Alloy 210.Alloy 210 is a ferritic alloy and possesses a fine-grained structure quite similar to that of alloy 196. In contrast to alloy 196, this structure was obtained through an addition of misch metal, not carbon. The machinability characteristics were improved to the same degree as alloy 196. The ductility remains the same and 5 further emphasizes the fact that a fine-grained structure doe not improve the ductility of territic iron-aluminum alloys to a significant degree. There are two very interesting facts about this alloy. The room-temperature tensile strength of a quenched specimen is the highest (116,250 p.s.i.) of any 100 percent ferritic alloy tested in the program. The other interesting fact is that there is little difference in tensile strength between the furnace-cooled and the quenched specimens. Misch metal probably has some efiect upon either the ordering process, itself, or upon the rate of the ordering process.

The 1200 F. tensile strength is quite poor (29,500 p.s.i.) when compared to other alloys. The misch metal improves the room-temperature tensile strength more than carbon does, but at 1200 F. carbon improves the tensile strength more than misch metal.

Alloy 213.'lhis is one of the few completely austenitic alloys obtained in this program. The chief dilierence between this alloy and alloy 179, discussed previously, is that this alloy contains 0.76 percent carbon and 10.2 percent aluminum. The room-temperature tensile properties are excellent: 110,750 p.s.i. tensile strength, 72.5 percent elongation, and 82 percent reduction in area. The ductility is two orders of magnitude higher than the completely ferritic iron-aluminum alloys, and the tensile strength is superior to most ferritic iron aluminum alloys. The machinability characteristics are very good, comparable to that of a 304 stainless steel. There is little dittference between the tensile strengths of quenched and furnace-cooled specimens, but a large difierence in yield strengths. The specimens fracture in the cup-cone manner of a ductile alloy, and necking of the specimen takes place during the latter stages of the room-temperature test.

The 1200" -F. tensile strength is excellent and the value of 60,500 p.s.i. is only surpassed by two alloys, 196 and 172. After testing, the specimen was slightly magnetic, which probably means a small amount of ferrite precipitated during the actual test. This did not occur in any of the room-temperature tests, as the alloy was non magnetic before and after each test.

Alloy 214.-T his alloy is the third completely austenitic alloy subjected to tensile testing. This alloy is similar to alloy 213, having an aluminum concentration of 7.5 percent instead of 10.2 percent, and almost identical to alloy 179 except for the carbon concentration. The room-temperature tensile properties are excellent: 115,000 p.s.i. tensile strength, 62.5 percent elongation, and 73.5 percent reduction in area. The high carbon concentration (-0.79 percent) increased the room-temperature tensile strength by 45,000 p.s.i. over alloy 179, which has a low carbon concentration (0.03 percent). The high carbon concentration had no adverse effect on the ductility as it still remains almost two orders of magnitude higher than the ferritic alloys.

The 1200 F. tensile strength is very good, but slightly inferior to that of alloy 213. This is probably due to the difference in aluminum concentrations. The elongation at this temperature is higher, probably because no ferrite precipitated during the actual test. This alloy was completely non-magnetic after the 1200 F. test.

Alloy 226.This alloy and alloys 227 and 228 are attempts to improve the oxidation resistance of austenitic iron-aluminum alloys with additions of nickel. Alloy 226 contains 2.3 percent nickel, 10 percent aluminum, 33.8 percent manganese, and 0.27 percent carbon. This alloy, along with alloys 227 and 228, is more magnetic after quenching than upon furnace cooling. The roomtemperature tensile results of alloy 226 are quite good when compared to other alloys. The tensile strength of 119,000 is good, and the ductility is only somewhat less than that of the completely austenitic alloys. There is a difference between the furnace-cooled results and the quenched results, the furnace-cooled specimens having lower tensile strengths accompanied by lower ductility.

The 1200" F. tensile strength is average, and the ductility should be considered as normal.

Alloy ZZZ-This alloy is quite similar to alloy 226 except that it contains 4.3 percent nickel and the aluminum concentration is somewhat higher (12.5 percent). The room-temperature tensile strength is the same as alloy 226, but the tensile strength of the furnace-cooled specimens shows a slight rise. The room-temperature ductility remains about the same for both the furnace-cooled and the quenched specimens. The 1200 F. tensile strength is the same as that of alloy 226, except the ductility is less.

Alloy 228.This alloy contains 6.4 percent nickel and 9.3 percent aluminum. The tensile strength, although only slightly higher than alloys 226 and 227, is considered to be very good (123,000 p.s.i.). This tensile strength is the third highest obtained in the testing program. The ductility is only half that of alloy 213, but is still one order of magnitude higher than the ferritic ironaluminum alloys. Although this alloy is magnetic upon quenching, it is considered to be only slightly magnetic upon furnace cooling. The 1200" F. tensile strength is very good, and the ductility associated with it is normal.

Density.Density measurements were made and are given in Table 4.

TABLE 4 Alloy No. and Treatment Density gm./ cc. 173 forged 6.59 213 forged 6.69 213 hardened 6.59 228 forged 6.79 1020 steel 7.86 18.8 stainless steel 7.93 S816 cobalt base 8.59 Hastallog B 9.24

It is interesting to note that the density values are about 15 percent lower than that of stainless steel and much lower than that of high temperaure alloys.

Oxidation resistance.0xidation tests were run on most of the alloys that were subjected to the tensile testing program and on a few selected commercial alloys. Small wafers 0.125-inch thick were used as specimens and after the oxidation test the thickness of metal that remained microscopically unaffected was measured metallographically. The oxidation tests were carried out at 1500 F. for 100 hours and 2200 F. for 46.5 hours, and the results are given in Table 2.

TABLE 2.OXIDATION TEST [Depth of oxide penetration in inches] Alloy No. 1,500 F.- 2,200 F. 100 hours 46.5 hours 0005 0005 0005 0005 0005 0015 0005 0005 0005 002 0005 001 0005 000 5 0065 014 0155 047 001 0075 O02 0035 .004 005 .0005 0025 8-816 (cobalt ba 0005 .0625 U-500 (nickel base) .0005 .0025

The oxidation resistance of the experimental alloys as a whole is excellent at 1500 F. Only one alloy (214) oxidized excessively and three alloys (213, 227, and 228) oxidized moderately. These four alloys contained the largest amounts of manganese and carbon per unit concentration of aluminum.

The oxidation resistance of the experimental alloys as a whole is good at 2200 R, with only two experimental alloys (213 and 214) oxidizing excessively. As can be seen from Table 5, most of the alloys that oxidized moderately were superior to the three commercial alloys tested (310 stainless steel, 8-816, and Udimet 500). Three experimental alloys (154 172, and 210) possessed oxidation resistance at 2200 F., equal to that of alloy 133.

Cold Rllz'ng.ortions of the three best alloys (173, 213 and 228) were forged into /2 inch square rod for ease of handling prior to rolling. The forged rod was cut into six sections and five of the six sections were hot rolled at 2000 F. to live different thicknesses; 0.250, 1.167, 0.125, 0.111, and 0.125 inch. One section of each ingot was not hot rolled. All sections were then annealed /2 hour at 2000 F. to eliminate the effects of any cold work that might be present from rolling. All sections were then cold rolled to the same thickness (0.100 inch).

No difficulty was encountered in cold reducing the 100 percent and 80 percent austenitic alloys by normal steel processing techniques. These two alloys were cold reduced up to 80 percent with reductions of 20 percent per pass being used. The alloy containing 40 percent austenite could be reduced 66 percent using the techniques mentioned above and this could be compared to binary iron-aluminum alloys, which can be cold rolled only at very light gage and by the most careful rolling techniques.

Recrystallizatiom-Jn an effort to determine the recrystallization temperature, portions of each of the coldreduced sections were annealed at seven temperatures, ranging from 1292 F. to 1832 F. at 90 F. intervals, for one hour, then water quenched. Rockwell A hardness, grain size, and free angle of bend were determined. The hardness results are depicted graphically in the drawing.

An unexpected hardening effect was found when the specimens were heat treated at 1292 F. for 1 hour, which is readily seen in the drawing. The hardness of alloy 213 is increased seven points over that of the cold-rolled strip after 80 percent reduction. This hardness is comparable to the hardness of a martensitic alloy before it is ternpered. The importance of this phenomenon cannot be over emphasized. It means that the alloy can be rolled easily into sheet, then hardened to a considerable degree by heat treating. In View of these results, the tensile-test and stress rupture tests should be thought of as being performed on annealed material and, therefore, should not be considered as the maximum values attainable.

Upon close examination of the drawing, it is seen that this hardening phenomenon is observed in all three alloys tested and increases in strength as the amount of austenite increases. The hardening eifect is only observed on alloys reduced at least 20 percent and, in general, the effect increases as the amount of cold work increases. The effect of cold work and the hardening phenomenon are still present when the specimens are annealed as high as 1560 F. Annealing or heat treating temperatures between about 1110" F. and about 1740 F. are preferred.

Taking all things into consideration, the recrystallization temperature was determined to be 1750 F. for alloy 173 and 1550 F. for alloys 213 and 228. Both these recrystallization temperatures are considerably above the recrystallization temperature (1400 F.) for binary ironaluminum alloys.

It can be seen from the above discussion, by changing the crystal structure from body-centered-cubic (ferrite) to face-centered-cubic (austenite) that not only was the room temperature and 1200 F. tensile strength increased, but the room-temperature ductility was greatly improved. Austenite increases the ductility to a large degree and to some extent the tensile strength. The ferritic alloys studied exhibited poor room temperature ductility even the fine grained alloys.

The amount of austenite present in an alloy was found to be the significant factor, the ductility increasing as the amount of austenite increased. Carbon was also found 1% to be a significant factor in increasing room temperature and 1200 F. tensile strength. Carbon concentrations between 0.15 and 1.1 percent are preferred to obtain the best tensile strengths without adversely effecting the ductility or machinability of the alloys containing at least 15 percent austenite. In addition, carbon not only increases the amount of austenite but also in appreciable concentrations refines the grain structure to a considerable degree.

The feiritic alloys exhibited quite a few problems in machining, fabricating and heat treating while the alloys containing austenite in amounts greater than 15 percent are quite easily handled and behave quite similar to commercial stainless steels.

The manganese and carbon limitations are quite critical. At concentrations of manganese above 40 percent by weight of the alloy, it ha been found that the alloy readily crumbles or falls apart. Manganese concentrations below about 20 percent by weight will not produce an alloy with the required amount of austenite, i.e., 15 percent or better required to obtain the excellent properties. Carbon in amounts greater than about 1.1 percent by weight lead to difiiculties in machinability and greatly decrease the oxidation resistance of the alloy. Alloys with carbon concentrations below 0.15 percent by weight have lower tensile strengths and are more coarse grained "structures which presents difiiculties of machinability.

Aluminum tends to form body centered cubic (ferritic) alloys while manganese and carbon tends to form facecentered-cubic (austenitic) alloys. As the percentage of aluminum in the alloys increases, then so must the perceutage of manganese if an alloy containing at least 15 percent austeni te is to be obtained. As the aluminum content is increased, so can the carbon content so as to aid in formation of the desired austenite phase. Thus the amounts or concentrations of aluminum, manganese and carbon must be suitably proportioned and closely controlled within the limits recited to produce the present alloys which possess excellent properties and which are a substantial improvement :over the heretofore known binary iron-aluminum alloys or modified versions thereof.

Since certain changes can be made in the above alloys and processes without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description and drawing should be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A low density, ductile, high strength, high temperature and oxidation resistant alloy consisting essentially of by weight 2 0 to 40 percent manganese, 7 to 16 percent aluminum, 0.15 to 1.1 percent carbon, up to 10 percent nickel and the balance essentially iron.

2. A low density, ductile high strength, high temperature and oxidation resistant alloy consisting essentially of by weight 20 .to 40 percent manganese, 7 to 16 percent aluminum, 0.15 to 1.1 percent carbon, and the balance essentially iron.

3. A low density, ductile, high strength, high temperature and oxidation resistant alloy consisting essentially of 20 to 40 percent manganese, 7 to 16 percent aluminum, 0.15 to 1.1 percent carbon, 2 to 7 percent nickel and the balance essentially iron.

4. A low density, ductile, high strength, high temperature and ox dation resistant alloy consisting essentially of by weight 20 to 40 percent manganese, 7 to 16 percent aluminum, 0.15 to 1.1 percent carbon and the balance essentially iron, said alloy being at least 15 percent austenite.

5. A low density, ductile, high strength, high temperature and oxidation resistant alloy consisting essentially of by weight about 15 percent aluminum, about 1.1 percent carbon, about 28 percent manganese and the balance essentially iron.

6. A low density, ductile, high strength, high temperature and oxidation resistant alloy consisting essentially of by weight about 12 percent aluminum, about 0.7 percent References Cited in the file of this patent UNITED STATES PATENTS Hadfield Mar. 4, 1890 Mitchell et al. Dec. 27, 1932 Mishima Jan. 14, 1936 Dean July 27, 1943 Dean May 29, 1945 

1. A LOW DENSITY, DUCTILE, HIGH STRENGTH, HIGH TEMPERATURE AND OXIDATION RESISTANT ALLOY CONSISTING ESSENTIALLY OF BY WEIGHT 20 TO 40 PERCENT MANGANESE, 7 TO 16 PERCENT ALUMINUM, 0.15 TO 1.1 PERCENT CARBON, UP TO 10 PERCENT NICKEL AND THE BALANCE ESSENTIALLY IRON. 